1. Field of the Invention
One or more embodiments relate to a method of operating a fuel cell, and more particularly, to a method of removing residual oxygen from a stack of a residential high temperature non-humidification fuel cell.
2. Description of the Related Art
Residential fuel cell systems are, in general, operated in daily start-up and shut-down (DSS) cycles. When a residential fuel cell system is stopped or shut down, nitrogen purging is used to reduce the cell voltage below a predetermined level in order to prevent corrosion of the carbon support of a cathode or cathodes because of the excessive voltage.
Nitrogen purging needs a nitrogen bomb and an additional pipe and valve. Thus, the size and number of components of residential fuel cell systems increase. However, nitrogen purging makes it difficult to remove residual oxygen in the micro-pores of an electrode. Thus, after nitrogen purging is completed, the residual oxygen may continue to cause corrosion of carbon.
When residential fuel cell systems are temporarily stopped, that is, when residential fuel cell systems are in an idle mode, the residual oxygen may be removed from a fuel cell stack by nitrogen purging (substitution by nitrogen). In addition, the residual oxygen is removed from the fuel cell stack by using two methods: by charging a cathode with hydrogen created by the electrolysis of water and sealing the cathode, and by interrupting air supply into the fuel cell and removing the residual oxygen by using a constant current.
The first method may increase the removal rate with respect to the residual oxygen. However, the first method may not be easily applied to a high temperature non-humidification fuel cell system (a fuel cell that operates at a high temperature under non-humid conditions), may lead to hydrogen leakage and may lead to oxygen being supplied into the fuel cell again.
The second method may produce a negative pressure lower than atmospheric pressure inside the cathode due to a reduction of partial pressure caused by the removing of the residual oxygen, which may lead to air being supplied into the fuel cell from outside. Furthermore, a side reaction may occur due to a local concentration of current density.